Low-intensity exercise reverses chronic muscle pain in the rat in a naloxone-dependent manner

Objective: To determine the effects of low-intensity exercise on chronic muscle pain and potential activation of the endogenous opioid system. Design: Randomized placebo-controlled trial. Setting: Animal laboratory. Animals: Sixty-three male Sprague-Dawley rats. Interventions: Rats performed a low-intensity exercise protocol for 5 consecutive days after the induction of chronic muscle pain. In a separate experiment, naloxone or saline was administered systemically before 5 low-intensity exercise sessions. Main Outcome Measure: Mechanical hyperalgesia was measured using von Frey filaments to determine the mechanical withdrawal threshold. Results: Low-intensity exercise increased mechanical withdrawal threshold in the chronic muscle pain model. Naloxone attenuated the antihyperalgesic effects of low-intensity exercise. Conclusions: Low-intensity exercise reversed mechanical hyperalgesia in the chronic muscle pain model through activation of the endogenous opioid system.